A variety of techniques can be utilized to image scenes in ways that capture information within different portions of the visible and/or electromagnetic spectrum. ‘Color filters’ are often used with a camera system to filter out portions of the electromagnetic spectrum with the exception of a specific band, such that only the particularly exempt spectral band is able to transmit through the filter. Thus, for example, a red color filter typically operates to filter out all portions of the electromagnetic spectrum except for the band corresponding with visible red light. Color filters are often implemented as patterns of filters applied to individual pixels on an image sensor. A common example of a filter pattern is the Bayer filter pattern. A Bayer filter pattern typically includes an array of red, green, and blue color filters intended to be disposed over a grid of photosensors (e.g. pixels or photosites), where each color filter is associated with a single photosensor. In a Bayer filter, there are usually twice as many green color filters as there are red or blue color filters, which is meant to mimic the physiology of the human eye. In a Bayer filter configuration, each respective photosensor is intended to obtain imaging information concerning a particular band of the electromagnetic spectrum. The aggregate of the imaging information can thereafter be ‘demosaiced’ or interpolated to produce a color image. Note that the term ‘color filters’ can also be applicable with respect to those portions of the electromagnetic spectrum adjacent to the visible light portion, e.g. the ultraviolet and infrared portions of the electromagnetic spectrum.
The quality of an image captured by an imaging system is typically dependent upon the number of photons incident on the pixels of an image sensor. A variety of techniques can be utilized to increase the intensity of light incident on an image sensor including increasing the size of the optics to capture more light and/or increasing the integration or exposure time of the pixels. The extent to which exposure time can be increased is often limited based upon relative motion between the imaging system and the scene and/or motion within the scene itself. As exposure time increases, scene motion can introduce motion blur artifacts into the resulting image.
Time delay integration (TDI) is an imaging technique that is typically implemented in conjunction with charge-coupled device (CCD) image sensors for imaging systems that move in a predictable way relative to a scene. CCD image sensors typically operate as follows: (1) a CCD image sensor typically includes a grid of pixels; (2) when an image of a scene is desired, electrical charge is stored in the grid of pixels as a function of the scene's light intensity; (3) the stored electrical charge is shifted—from one row of pixels to the next—until it reaches a serial register, where stored electrical charge corresponding with each pixel then proceeds to be read out and stored as image data. Note that in a conventional CCD imaging technique, each pixel in the grid of pixels stores light intensity information corresponding with a different aspect of the scene.
A TDI mode of operation can be useful when it is known that the scene to be imaged is moving in a known and predictable manner relative to the CCD image sensor. Whereas capturing an image in this scenario using a conventional CCD imaging technique can result in motion blur, capturing an image in this scenario using a TDI mode of operation can increase the exposure time of each resulting image pixel while mitigating the development of motion blur. In a typical TDI mode of operation, a scene being imaged is moving in an “along track” direction—corresponding with a column of pixels. Relatedly, the orthogonal direction is known as the “across track” direction—and it corresponds with a row of pixels. FIG. 1 diagrams this configuration. In effect each row of pixels ‘scans’ a scene such that light from a point in the scene impinges on each successive pixel of a given corresponding column. As the light impinges on each successive pixel, corresponding electrical charge is stored within the respective pixel, and the stored electrical charge is cumulatively added to each successive pixel, such that the summation is indicative of all the light that impinged on the column. As can be appreciated, because light from a single point within a scene impinges on the image sensor for a greater duration using a TDI mode of operation relative to a conventional CCD imaging technique, a TDI mode of operation can produce an image characterized by a relatively higher signal to noise ratio. In other words, TDI imaging techniques can provide for a longer exposure time since light is being aggregated due to a point in a scene being successively focused onto multiple pixels in a column of pixels due to relative motion of the camera system and the scene, allowing for more photons to be detected, and this can result in a significantly higher signal to noise ratio.